Many different methods for the preparation of epoxides have been developed. Generally, epoxides are formed by the reaction of an olefin with an oxidizing agent in the presence of a catalyst. The production of propylene oxide from propylene and an organic hydroperoxide oxidizing agent, such as ethyl benzene hydroperoxide or tert-butyl hydroperoxide, is commercially practiced technology. This process is performed in the presence of a solubilized molybdenum catalyst, see U.S. Pat. No. 3,351,635, or a heterogeneous titania on silica catalyst, see U.S. Pat. No. 4,367,342. Hydrogen peroxide is another oxidizing agent useful for the preparation of epoxides. Olefin epoxidation using hydrogen peroxide and a titanium silicate zeolite is demonstrated in U.S. Pat. No. 4,833,260. One disadvantage of both of these processes is the need to pre-form the oxidizing agent prior to reaction with olefin.
Another commercially practiced technology is the direct epoxidation of ethylene to ethylene oxide by reaction with oxygen over a silver catalyst. Unfortunately, the silver catalyst has not proved useful in commercial epoxidation of higher olefins. Therefore, much current research has focused on the direct epoxidation of higher olefins with oxygen and hydrogen in the presence of a catalyst. In this process, it is believed that oxygen and hydrogen react in situ to form an oxidizing agent. Thus, development of an efficient process (and catalyst) promises less expensive technology compared to the commercial technologies that employ pre-formed oxidizing agents.
Many different catalysts have been proposed for use in the direct epoxidation of higher olefins. For example, JP 4-352771 and U.S. Pat. Nos. 5,859,265 and 6,008,388 disclose the production of propylene oxide from the reaction of propylene, oxygen, and hydrogen using a catalyst containing a Group VIII metal such as palladium on a crystalline titanosilicate.
Unfortunately, catalysts of the type disclosed above tend to slowly deteriorate in performance when used repeatedly or in a continuous process for a prolonged period of time. In particular, the catalyst activity decreases with time to a point where continued use of the catalyst charge is no longer economically viable. Due to the relatively high cost of synthesizing this type of catalyst, regeneration of the used catalyst would be greatly preferred over replacement.
U.S. Pat. No. 6,380,119 discloses a method of regenerating a zeolite, particularly a titanium silicalite, by a three-stage calcination process in which the temperature is varied from 250-800° C. and the oxygen content is varied over the three stages. Baiker et al., App. Catal. A: General 208 (2001) 125, discloses the washing of a used Pd—Pt/TS-1 catalyst to partially remove non-volatile organic residue from the catalyst. In addition, Baiker speculates that reactivation of the Pd—Pt/TS-1 catalyst requires an oxidative treatment at elevated temperatures, but that earlier work indicates that such a treatment would result in reduced catalytic performance. U.S. Pat. No. 5,859,265 also states that a palladium titanium silicalite catalyst may be regenerated by either a simple wash process or by a controlled burn at 350° C. followed by reduction.
As with any chemical process, it is desirable to develop new and improved regeneration methods. We have discovered an effective regeneration method to restore the activity of a used noble metal-containing titanium zeolite catalyst.